Jeff's DIY

Advice on doing your own diagnosis and repair

Advice from the book Can I Do It Myself?

Home HVAC Basics and Common Fault Repair

Modern home systems use a set of ducts to distribute the conditioned air from the centralized unit and another set of simpler ducts to return the air back to the unit. For a smaller ranch home, there may be only one return vent. In either case, the air is filtered to remove dust before going back into the unit. The gas-powered furnace may use a pilot light or spark ignition. All gas-powered systems have a vent for exhausting combustion products, though the exhaust system can vary significantly with the efficiency of the particular unit. In either case, there are usually sensors used to ensure the combustion products do not back up into the home. There is always also a flame sensor to ensure the combustion happens, or the board will shut off the gas supply. The heart of the unit is the heat exchanger, where the air passes through heated or cooled baffles to be conditioned. A unit that has A/C will have a second heat exchanger, called the evaporator, to cool the air. The other primary component is the blower. There are often also other sensor or micro-switches used for additional safety measures, such as one to prevent the blower from operating when the door is off the blower compartment. In the following paragraphs, I will identify some common fault conditions that you may be able to repair yourself.


System ignites but then shuts off within a few seconds.

This is a very common problem and is almost always caused by the combustion flame not being sensed by the control board. This can also happen to the pilot light, if the unit uses a pilot light. There are several possible faults that yield this result. The easiest and most common is an oxidized sensor. Locate the sensor in the path of the flame or gas. It is usually a metal cylinder with one wire coming off the back. If you see any buildup on the sensor, shut off the thermostat and remove the oxidation with emery cloth or sand paper of a medium grit (150 or higher). If you removed the sensor or moved it, make sure you put it back into the flame path before turning the system back on. If this does not fix the problem, you either need a new sensor or a new control board. Assuming everything else is working OK and there is no obvious damage to the control board, I would next replace the sensor. If you have a repair manual for the furnace, you can test the sensor output to see if it is faulty before buying a new one. If the system still fails with a new sensor, I would replace the board.


Furnace is triggered by the thermostat and exhaust inducer fan comes on, but the gas valve never opens.

This fault is usually due to the flow sensor in the exhaust not getting to the control board. Try removing the leads from the sensor and connecting them together. If the sensor is bad, this should restore function. Replace the sensor to restore safe function.


Furnace is triggered by the thermostat and exhaust inducer fan comes on, but the gas valve never opens.

If the inducer motor does not come on at all, the motor is usually at fault and will not allow the furnace to run, as it believes the exhaust is blocked. You might be able to test the rest of the system and isolate the fault to the inducer fan by shorting the flow sensor leads together.


Combustion or outside A/C unit works fine, but the blower does not come on.

This fault may be caused by a failed blower motor/condenser, control board relay or blower door micro-switch. Check the relay first by triggering the system and wait to see if the relay clicks. There is always a delay between the system being triggered and the motor starting to allow the heat exchanger to heat or cool ahead of the air. If relay clicks, the board is OK. Check the blower door switch next by pulling the leads off the switch and shorting them together. Trigger the system and wait to see if the blower starts. If the blower starts, replace the switch. If the blower does not start, further troubleshooting will require the repair manual for the system. If you have the manual, you may be able to test the motor by closing the relay manually or by whatever procedure the manual may recommend. I do not recommend replacing either of these components without a complete diagnosis, as these parts are usually expensive.


Blower comes on but then shuts off within a few seconds.

For systems that use an airflow sensor in the primary stream, this fault is caused by a failed flow sensor or clogged filter. Check the filter first. If it is clogged, try operating the system without a filter. If it works without a filter, replace the filter. If not, see if you can find the flow sensor, pull the leads off and short them together. If that works, replace the flow sensor. If neither the filter nor switch is at fault, the problem is likely the control board.


Air Conditioner air is not as cold as it used to be.

This condition is usually due to a leak in the system that has allowed refrigerant to escape and in turn reduces the effectiveness of the system. If it is legal where you live, you may be able to charge the system with a set of gages and a refrigerant bottle, but that is a lot of equipment to buy; and you will need even more equipment if you want to find and fix the leak. For this reason, I have never done this kind of work myself. There is one other case I have encountered that I was able to repair by myself, and this is the case in which the condenser cooling fan was not running at full speed. This may be caused by a failed capacitor or motor. The fan may turn slowly or not at all. The capacitor is a silver cylinder near the fan usually covered by a panel and is usually inexpensive. The condenser will have two or four electrical connections (depending on whether it runs only the fan or also the compressor motor) and may show a bulge if it has failed. Make sure to shut of the A/C circuit breaker and then trigger the unit to discharge the capacitor before touching the leads.

Heat Pumps

A heap pump is essentially a reversible air conditioner. When in cooling mode, it works just like an air conditioner. When in heating mode, the refrigerant flow is reversed. This requires either a DC motor, a gearbox, or valving to reverse the flow direction. Other than that, a heat pump is the same as an air conditioner. Heat pumps have been around a long time, but they only save money in certain regions of the country. In cold winters, they may not be able to adequately heat the home, thus requiring the use of additional portable heaters.

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